1. Field of the Invention
The present invention relates to a biaxially oriented polypropylene film to be metallized, and a metallized biaxially oriented polypropylene film, and a laminate formed by using it.
In more detail, the present invention relates to a biaxially oriented polypropylene film to be metallized, excellent in film formability, high in stiffness, and excellent in the adhesiveness to a metallic film and likely to have excellent metallic gloss after metallization, and a metallized biaxially oriented polypropylene film excellent in gas barrier properties and also excellent in heat resistance and stiffness, hence also excellent in the gas barrier properties after processing such as lamination. The present invention also relates to a laminate for packaging using said film. Furthermore, it relates to a metallized biaxially oriented polypropylene film excellent in the adhesiveness between the thin metallic film and the base material, hence free from the disadvantage called pick-off that the thin metallic film is transferred to the side opposite to the metallized side even though the side opposite to the metallized side is excellent in adhesiveness and heat sealability.
2. Description of the Related Arts
Biaxially oriented polypropylene (BOPP) films are widely used as films for packaging, since they are excellent in moisture barrier properties, strength, clarity and surface gloss, and it is also widely practiced to metallize them by a metal such as aluminum, for the purposes of making them look better because of the metallic gloss when displayed, improving the gas barrier properties, and inhibiting the deterioration of the packaged product otherwise caused by external light such as ultraviolet light.
However, since the surface of a BOPP film is inactive, it is generally practiced to activate the surface by such treatment as corona discharge treatment or flame treatment, for improving the adhesiveness between the metallic film and the base material film at the time of metallization.
To improve the effect of corona discharge treatment, U.S. Pat. No. 4,297,187 discloses corona discharge treatment in a mixed gas consisting of nitrogen and carbon dioxide. However, in the corona discharge treatment method, a higher treatment intensity can activate the surface more and improves the adhesive strength between the metallic film and the surface layer of the base material film, but it is known that, at the same time, the base material is deteriorated and therefore that the surface layer of the base material film is likely to be delaminted from the inside of the base material film. So, there is a limit in the effect of improving the adhesive strength. Furthermore, it is also known that if the treatment intensity is raised too much, blocking is likely to be caused.
U.S. Pat. No. 4,345,005 discloses a metallized BOPP film obtained by forming an ethylene-propylene copolymer resin layer containing about 2% to 4% of ethylene at least on one side of a base layer made of isotactic polypropylene resin by co-extrusion, treating the resin layer by corona discharge, and metallizing it.
Furthermore, U.S. Pat. No. 4,357,383 discloses a metallized multi-layer film for packaging, in which a metal layer is formed on a random copolymer layer consisting of ethylene and 0.25 to 15 wt. % of an .alpha.-olefin with 3 to 6 carbon atoms formed on a base layer. Similarly as a biaxially oriented polypropylene conjugated film to be metallized by a metal oxide, Japanese Patent Laid-Open (Kokai) No. 9-94929 discloses a film in which the heat of crystal fusion of the polyolefin resin formed as the surface layer to be metallized is 30 to 85 J/g, and proposes polypropylene copolymers, syndiotactic polypropylene resin, ethylene-a-olefin copolymers, blends consisting of any of these resins and isotactic homopolypropylene or propylene copolymer, as the resins satisfying the condition.
Moreover, Japanese Patent Laid-Open (Kokai) Nos. 6-67285 and 6-126281 disclose that the surface layer to be metallized is formed by syndiotactic polypropylene or a mixed resin consisting of syndiotactic polypropylene and isotactic polypropylene.
Of these surface layer resins, if a copolymer resin is laminated as a surface layer, the adhesiveness between the metallic film and the surface layer resin can be improved. However, since a copolymer resin is generally low in melting point, for example, it sticks to longitudinal stretching rolls at the time of film formation, to significantly restrict film formation, and the decline of gloss by sticking marks also poses a problem. Furthermore, because of the low melting point, if the film is metallized, the heat of metal condensation and the radiation heat from the evaporation source are likely to whiten the metallic film, to pose a problem that it is difficult to obtain the intended metallic gloss. Similarly, if the surface layer is formed by syndiotactic polypropylene, it is known that the melting temperature declines as stated in Japanese Laid-Open (Kokai) No. 7-89022, to pose a problem of heat resistance as in the case of copolymer resins.
U.S. Pat. No. 4,419,410 discloses a technique for an oriented polypropylene film with relatively low stereoregularity polypropylene laminated on high stereoregularity polypropylene to promote the manifestation of an organic slip agent and an anti-static agent. However, as described in said US Patent No. 4,345,005 and Japanese Patent Publication (Kokoku) No.8-18404, it is known that particularly the organic slip agent added as one of these additives worsens the adhesiveness to the metallic film, and the technique cannot be applied for a BOPP film to be metallized.
U.S. Pat. No. 4,888,237 discloses a film obtained by applying flame treatment to a surface layer containing at least 50 wt. % of an isotactic homopolymer of not more than 10 wt. % in xylene soluble content and metallizing the surface layer. The isotactic homopolymer is ordinary propylene homopolymer with an isotacticity of substantially not more than 6%, and it is stated to the effect that in the case of propylene homopolymer with an isotacticity of 6% to 15%, it is preferable to contain it up to 50%. It is disclosed that the use of such a polymer in combination with flame treatment greatly improves the adhesiveness.
However, it is stated that corona discharge treatment does not show any effect of improving the adhesiveness, and there has been no surface layer resin which can be improved in adhesiveness even by simple corona discharge treatment.
Important properties of a metallized BOPP film using said base material include the gas barrier properties to prevent the deterioration of the packaged product otherwise caused by oxygen and water vapor in the use of it as one of packaging materials, by metallization to lower the oxygen transmission rate and water vapor transmission rate. Since the gas barrier properties greatly affect the shelf life of food as a main packaged product, a metallized BOPP film with higher gas barrier properties is being demanded.
Metallized polypropylene films obtained by using low melting point resin laminated polypropylene films as prior arts have a problem that when the metallized side and another material (such as a printed BOPP film or biaxially oriented polyester film) are bonded to each other for lamination, using extruded polyethylene, etc., the low melting point resin makes the metallic film lose its metallic gloss or greatly lowers the gas barrier properties disadvantageously.
The metallized BOPP films are, for example, used in the following ways. The film is coated with an adhesive on the portions to be sealed, on the side opposite to the metallized side for cold seal pouch making. As another method, polyethylene or polypropylene is melt-extruded for bonding, or a cast polypropylene film is bonded using an adhesive, and the bonded layer is used for heat seal pouch making. In these cases, the adhesiveness of the side opposite to the metallized side is important, and for the adhesiveness, it is preferable that the wetting tension on the opposite side is higher. However, if the wetting tension is too high, the thin metallic film is transferred onto the adhesive back side when the metallized film is wound as a roll, and this phenomenon called pick-off is not preferable in view of appearance and greatly lowers the gas barrier properties disadvantageously. So, it has been practiced to laminate an ethylene-propylene block copolymer layer or a layer containing inorganic particles on the side opposite to the metallized side for highly roughening the surface as a means for decreasing the pick-off. However, even these methods have a problem that if the wetting tension of the back side is too high, the problem of pick-off arises.
Furthermore, as another application of metallized BOPP films, it is practiced to laminate a polyolefin resin on the side opposite to the metallized side, for securing heat sealability. However, the metallized BOPP film using a low melting point resin layer of any prior art still cannot overcome the phenomenon of pick-up that the thin metal layer is transferred onto the heat sealable layer on the back side.